Clean up error messages and assertions

[dajmcdon]

• Add helper functions to validate f_args
• Remove the Abort / Warn plugins
• Replace all warnings/errors with functions from {cli} and appropriate error messages.

[dshemetov]

@dajmcdon do we still need (1) if we're using checkmate?

[dajmcdon]

Possibly. Or, maybe 1 simply gets subsumed with 2.

[dshemetov]

Got it. I'm starting to notice a lot of hand-written arg validation functions like:

validate_version_bound <- function(version_bound, x, na_ok,
                                   version_bound_arg = rlang::caller_arg(version_bound),
                                   x_arg = rlang::caller_arg(version_bound)) {

I'm thinking we can replace these with checkmate calls.